Carbon nanotubes (abbreviated below as “CNTs”) are structures that can be counted on to have a high electrical conductivity, and are being studied for potential use in a wide range of fields as a key nanotechnology material.
In the actual use of such CNTs, to achieve a high conductivity in a smaller amount, the CNTs must be uniformly dispersed within a polymer or the like that serves as a matrix material.
Dispersion methods can be broadly divided into methods that involve modifying the carbon nanotubes themselves so that they easily disperse in the matrix material, and methods that involve the use of a dispersant such as a surfactant or a polymer.
Of these, methods involving the use of a dispersant are commonly employed because the carbon nanotubes can be uniformly and highly dispersed while retaining their electrical conductivity.
Various dispersants, ranging from small molecules to polymers, are being investigated, although small-molecule dispersants generally have a low dispersibility and a poor heat resistance.
On the other hand, because polymeric dispersants have a low solubility, ionic functional groups often are deliberately introduced onto the molecule. Moreover, many such dispersants have aromatic rings in order to increase interactions with the CNTs.
Techniques for introducing functional groups onto CNTs themselves by chemical treatment, heat treatment or the like are being attempted, although this leads to new problems such as fragmentation of the CNTs and a decline in the electrical conductivity due to breakup of the conjugated system.
From another perspective in dispersant development, additional categories of CNTs dispersants include conjugated dispersants such as conductive polymers, and insulating non-conjugated dispersants.
When a conductive polymer is used, the conductivity properties of the resulting composition are improved, but this is accompanied by drawbacks; namely, a decrease in transparency due to light absorption by the dispersant, and the color tone.
On the other hand, in the case of non-conjugated dispersants, because these are insulators themselves, the dispersant is a factor that increases the contact resistance of the CNTs, preventing the conductive properties inherent to the CNTs from being fully manifested.
It is also known that doping CNTs with an inorganic acid, an organic acid, thionyl chloride or the like increases the conductivity of the CNTs (see, for example, Patent Document 1).
Yet, when such dopants are added to a CNT dispersion, the dispersed state of the CNTs sometimes becomes unstable due to changes in the pH of the dispersion or to interactions by the dopant with the CNTs, the dispersant and the dispersion medium.
Hence, investigations are also being widely carried out on the doping of conductive compositions obtained by removing the dispersion medium from a CNT dispersion.
However, in such cases, in addition to the problem that dopant does not fully penetrate to the interior, further drawbacks include an increase in the costs associated with the process, and the desire for a high chemical stability in the base material used because of a high reactivity of dopants.